This invention relates generally to the characterization of light emitting diodes, and specifically to a method of color sense and estimation for a red-green-blue triad of LED illuminants.
As low-cost, energy-efficient, solid-state light emitting diodes (LEDs) become increasingly available, particularly with recent improvements in the availability of low-cost blue LEDs, the use of LED arrays for illumination purposes is becoming more prominent. With the use of LEDs as illuminants comes the need to control the mixing of the substituent colors in order to attain and sustain a particular or desired type of color. As is well known, a triad of nominally red, green and blue light sources may be additively combined to give the perception of nearly any desired hue, saturation and luminosity, including an achromatic white light. A triad of nominally red, green and blue LEDs may be used to generate the desired color and intensity by varying the amount of current applied to each of the three LEDs. The resulting color then may be measured, and these measurements may be used to regenerate the color at a later time, or used in a system to maintain the desired color point. Applications for LED illuminants may include backlit displays for personal digital assistants and hand-held gaming devices, and backlighting for portable communication equipment, LED flashlights, indoor lighting, outdoor lighting and road lighting.
Traditional methods of color measurement may use expensive equipment such as spectrophotometers, spectrocolorimeters, calorimeters, spectroradiometers, or radiometric colorimeters. A spectrophotometer may measure the reflection or transmission characteristics of a sample at different wavelengths over the desired band. A calorimeter may provide tristimulus values or the coordinates in a uniform color space, such as L*a*b* chromaticity coordinates, using several broad response filters to modify the light source of the instrument in an attempt to duplicate a CIE illuminant and standard observer combination. A spectroradiometer may provide a spectral power distribution over the desired band, while a radiometric colorimeter may provide tristimulus values or chromaticity coordinates for light emitting from a radiant source, such as a hot filament, a solid-state laser or an LED illuminant. Information on color measurement techniques, and many other aspects of color science, may be found in the text entitled Color for Science, Art and Technology, edited by K. Nassua, Amsterdam, The Netherlands: Elsevier Science B. V., 1998.
These methods of color measurement, although accurate and traceable, are relatively expensive, time consuming, and require specialized test setups. Many applications need a simple yet accurate method of color sensing, using a compact structure that can be used for measuring the color of LED illuminants. Ideally, a rapid method and a compact structure may be contained in the same package as the LED illuminants to provide continuous monitoring, color setting, and color correction.
Many standards are available for use in the determination and measurement of color. Chromaticity coordinates x, y may be defined in a color space, as described by the CIE 1931 Standard Observer recommendations. The 1976 CIE L*a*b* system provides color coordinates along a green-red axis, a yellow-blue axis and a luminosity axis. An alternative color system, the 1976 CIE L*u*v* system, is used often for assessing radiant sources. CIE standards are published by the Commission Internationale de I""Eclairage (CIE), CIE Central Bureau, Kegelgasse 27 A-1030, Wein, Austria. Other standards are prominently used in industrial and commercial applications, such as those by the American Standards for Testing and Materials (ASTM), 100 Bar Harbor Drive, West Conshohocken, Pa. 19428, including publications ASTM 0001 through ASTM 0036.
Color tristimulus values X, Y and Z express the perception of color in accordance with a standard observer. The X, Y and Z tristimulus values are numerals physically representing the integral over a specified range of wavelengths of light reflected from a sample and the spectral power distribution of the illuminant measured at a particular wavelength, modified by the CIE 1931 Standard Observer color functions. When the tristimulus values are known, the color points may be obtained in other color coordinate systems including CIE x, y chromaticity coordinates, CIE L*a*b*, CIE L*u*v* or other color spaces that use various linear and nonlinear transformations.
Further transformations and corrections may be made to accurately record, display and reproduce color for copy machines, scanners, color TVs and monitors, printers, and digital cameras. An example of transformations used for image processing in digital cameras or color scanners may be found in xe2x80x9cMethod of Processing an Image Signalxe2x80x9d (U.S. Pat. No. 6,278,533). Colorimetric systems are described in xe2x80x9cColorimetric System Having Function of Selecting Optimum Filter From a Plurality of Optical Bandpass Filtersxe2x80x9d (U.S. Pat. No. 5,986,767), xe2x80x9cColorimetric Imaging System for Measuring Color and Appearancexe2x80x9d (U.S. Pat. No. 5,850,472) and xe2x80x9cColorimeterxe2x80x9d (U.S. Pat. No. 4,402,611).
Accurate determination of color tristimulus values in a compact system may benefit from the careful selection of color filters and associated detectors. In general, it is difficult to manufacture and find a set of color filters that match the human color-matching functions. A method of accurately and rapidly transforming the filter outputs may require transformation matrices that minimize the amount of calculations while providing additional terms and additional responses for cases where more accuracy is desired. A general guideline for photodetector and color filter selection to improve color-sensing accuracy for a specific set of illuminants also would be beneficial.
The object of this invention, therefore, is to provide a method and a system for color sensing and estimation of LED illuminants, and to overcome the deficiencies and obstacles described above.
One aspect of the invention is a method for determining tristimulus values for light emitting diode illuminants. The method may include the steps of determining a localized transformation matrix based on a set of spectral sensitivity functions and a set of color-matching functions, measuring a set of color responses associated with light emitted from the light emitting diode illuminants, and determining an estimate of tristimulus values based on the localized transformation matrix and the set of color responses.
The light emitting diode illuminants may include one or more red, green and blue light emitting diode triads. The localized transformation matrix may be computed using a Taylor""s series expansion at a peak red wavelength, a peak green wavelength, and a peak blue wavelength associated with the red, green and blue light emitting diode triad.
The color responses may be measured with a red color filter, a green color filter and a blue color filter. The localized transformation matrix may include matrix elements associated with the color responses from the red, green and blue color filters. The color responses may be measured with red, green and blue color filters, and red, green and blue edge filters. The localized transformation matrix may include elements associated with the color responses from the red, green and blue color filters, and the red, green and blue edge filters. The color responses may be measured with red, green and blue color filters, red, green and blue edge filters, and red, green and blue band-stop filters. The localized transformation matrix may include elements associated with the color responses from the red, green and blue color filters, the red, green and blue edge filters, and the red, green and blue band-stop filters.
The method for determining tristimulus values for light emitting diode illuminants may further include the step of calculating a set of color point values from the estimated tristimulus values. The method for determining tristimulus values for light emitting diode illuminants may further include the step of calculating a set of chromaticity coordinates from the estimated tristimulus values in a device-independent color coordinate system, a pre-defined color coordinate system, or a user-defined color coordinate system.
Another aspect of the current invention is a light emitting diode illuminant tristimulus value generation system including a means for determining a localized transformation matrix based on a set of spectral sensitivity functions and a set of color-matching functions, a means for measuring a set of color responses associated with light emitted from the light emitting diode illuminants, and a means for determining an estimate of tristimulus values based on the localized transformation matrix and the set of color responses.
The system may include a means for calculating a set of color point values from the estimated tristimulus values. The system may include a means for calculating a set of chromaticity coordinates from the estimated tristimulus values in a device-independent color coordinate system, a pre-defined color coordinate system, and a user-defined color coordinate system.
Another aspect of the current invention is a computer usable medium including a program for determining tristimulus values of light emitting diode illuminants including computer program code to determine a localized transformation matrix based on a set of spectral sensitivity functions and a set of color-matching functions, computer program code to measure a set of color responses associated with light emitted from the light emitting diode illuminants, and computer program code to determine an estimate of tristimulus values based on the localized transformation matrix and the set of color responses.
The localized transformation matrix may include matrix elements associated with the color responses from a set of red, green and blue color filters. The localized transformation matrix may include matrix elements associated with the color responses from a set of red, green and blue color filters, and a set of red, green and blue edge filters. The localized transformation matrix may include matrix elements associated with the color responses from a set of red, green and blue color filters, a set of red, green and blue edge filters, and a set of red, green and blue band-stop filters.
The computer usable medium may include computer program code to calculate a set of color point values from the estimated tristimulus values. The computer usable medium may include computer program code to calculate a set of chromaticity coordinates in a device-independent color coordinate system, a pre-defined color coordinate system, and a user-defined color coordinate system.
The aforementioned, and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.